1. Field of the Invention
The present invention relates to an image forming apparatus into which a charging control device is incorporated. The charging control device includes a high-voltage generation circuit which applies an oscillating voltage to a charging member which is disposed while brought into contact with or close to an image bearing body, a direct-current voltage and an alternating-current voltage being superimposed to form the oscillating voltage; and a voltage control unit which controls a peak-to-peak voltage value Vpp of the alternating-current voltage to a target voltage.
2. Description of the Related Art
Recently, a charging control device in which a contact charging method is adopted is becoming the mainstream from the viewpoints of low-voltage process of lowering a charging control voltage applied to an image bearing body, reduction of ozone generated in charging control, and cost reduction. In the contact charging method, a roller type or a blade type charging member is disposed while brought into contact or close to a surface of the image bearing body, and an oscillating voltage in which a direct-current voltage and a alternating-current voltage are superimposed is applied to the charging member to evenly charge the surface of the image bearing body. At this point, the oscillating voltage is not limited to a sine wave, but any periodically-changing oscillation waveform such as a rectangular wave, triangular wave, and a pulsating wave can be used.
Japanese Laid-Open Patent Publication No. 63-149668 discloses a technique in which the following charging characteristics are exerted when the above-described contact charging method is adopted.
That is, when a peak-to-peak voltage value of the alternating-current voltage in the oscillating voltage is boosted, a charging voltage of the image bearing body is increased in proportion to the increase in peak-to-peak voltage value. A charging potential is saturated when the peak-to-peak voltage value reaches about double a charging start voltage of the direct-current voltage, and the charging potential is not changed even if the peak-to-peak voltage value is further boosted. In order to ensure evenness of the charging, it is necessary to apply the oscillating voltage having the peak-to-peak voltage not lower than double the charging start voltage in applying the direct-current voltage determined by various characteristics of the image bearing body. The charging voltage obtained at that time depends on a direct-current component of the applied voltage.
Japanese Laid-Open Patent Publication No. 2001-201921 discloses a charging control method, wherein the image bearing body is evenly charged by adjusting a discharge amount from the charging member to the image bearing body such that problems such as deterioration of the image bearing body, toner adhesiveness, and image deletion due to the discharge are not generated even if a resistance value of the charging member fluctuates due to an influence of an environment.
Specifically, the control is performed as follows. During a non-image formation period, an alternating current value passed from the charging member to the image bearing body is detected, when at least one alternating-current voltage whose peak-to-peak voltage is lower than double a direct-current threshold voltage Vth is applied to the charging member. The direct-current threshold voltage Vth is one at which the discharge is started from the charging member to the image bearing body when the direct-current voltage is applied to the charging member.
Then, alternating current value passed from the charging member to the image bearing body are detected, when at least two alternating-current voltages whose peak-to-peak voltages are not lower than double the threshold voltage Vth are applied to the charging member.
The peak-to-peak voltage of the alternating-current voltage which should be applied in forming the image is determined based on the plural alternating current values detected in each step, and whereby the alternating-current voltage is controlled such that the peak-to-peak voltage is maintained in forming the image.
More specifically, a peak-to-peak voltage-alternating current function F1 and a peak-to-peak voltage-alternating current function F2 are determined on a two-dimensional coordinate in which a horizontal axis is set to a peak-to-peak voltage while a vertical axis is set to an alternating current. The peak-to-peak voltage-alternating current function F1 expresses a line segment connecting an origin (0 point) and an alternating current value which is detected when the peak-to-peak voltage lower than double the threshold voltage Vth is applied to the charging member. The peak-to-peak voltage-alternating current function F2 expresses a line segment including at least two alternating current values which are detected when the peak-to-peak voltage not lower than double the threshold voltage Vth is applied to the charging member. A peak-to-peak voltage value which becomes an intersecting point of the line segments expressed by the functions F1 and F2 is determined as the peak-to-peak voltage of the alternating-current voltage which should be applied in forming the image.
However, in an epichlorohydrin-rubber charging roller used as the charging member, characteristics fluctuate largely depending on an environment such as temperature and humidity. The adoption of the conventional charging control method in the epichlorohydrin-rubber charging roller causes the following problems. At this point, the image bearing body having the diameter of 30 mm is formed by depositing an amorphous silicon photoconductive layer having a thickness of 20 μm. The charging roller is disposed in contact with the image bearing body with a pressing force of 1 Kgf.
That is, as shown in FIG. 2, in a low-temperature environment (low-temperature environment 1 of FIG. 2), an electric resistance value of the epichlorohydrin rubber is increased to slow down motion of conductive ions in the rubber, which decreases the charging potential.
Accordingly, in order to adjust the charging potential of the image bearing body to a stable target potential, it is necessary that the peak-to-peak voltage value Vpp 1 of the alternating-current voltage be maintained at a peak-to-peak voltage value Vpp2 larger than a peak-to-peak voltage value Vpp 1 of ambient temperature environment.
However, in an extremely-low-temperature environment (low-temperature environment 2 of FIG. 2) such as 0° C., the peak-to-peak voltage value cannot be adjusted to a predetermined target potential even if the peak-to-peak voltage value is increased. In such low-temperature environments, because the charging potential at the image bearing body does not reach the target potential, problems such as fog (phenomenon in which toner adheres slightly to a background except for the image) and uneven density (phenomenon in which unevenness of charging state is generated to cause a fluctuation in density) are generated in the image formed in the image bearing body.
The problems are generated in not only a monochrome image forming apparatus in which the black toner is used, but also a tandem-type full-color image forming apparatus in which image bearing bodies are arranged in series along a sheet conveyance belt or an indirect transfer belt according to the yellow (Y), magenta M), cyan (C), and black (K) colors.